The invention relates to a process for the preparation of xcex5-caprolactam, in which, in a first step (a), a compound having the general formula:
O=CHxe2x80x94(CH2)4xe2x80x94C(O)xe2x80x94Rxe2x80x83xe2x80x83(1) 
wherein R represents xe2x80x94OH, xe2x80x94NH2 or xe2x80x94ORxe2x80x2, and wherein Rxe2x80x2 represents an organic group with from 1 to 10 carbon atoms, is brought into contact with ammonia and hydrogen in a suitable solvent at elevated pressure and in the presence of a hydrogenation catalyst, to form a mixture of primary amino compounds and xcex5-caprolactam. This is then followed by a separate second step (b) in which the primary amino compounds are reacted to form xcex5-caprolactam.
Such a process is described in U.S. Pat. No. 4,730,041. This patent describes a process in which methyl 5-formylvalerate is first reacted with excess ammonia and hydrogen in the pressence of methanol as solvent, and in the presence of a catalyst, such as Raney-nickel, in the liquid phase at 80xc2x0 C. to yield a mixture of about 89% methyl 6-aminocaproate and about 3% xcex5-caprolactam. This mixture is subsequently heated to 225xc2x0 C. to yield 78% xcex5-caprolactam. The total concentration of all of the reactants in the different process steps was about 10 wt %.
A disadvantage of the process described in this U.S. Pat. No. 4,730,041 is that relatively large-sized process equipment for the (step (b)) cyclization section is required. This is due to the low concentration of reactants in that step. According to the patent the cyclization is also performed at super atmospheric pressures, requiring special process equipment. From an economical/investment point of view, smaller-sized, less expensive, process equipment is generally desired. However, merely using smaller-sized process equipment (which might otherwise seem possible simply by increasing the reactant concentration in the cyclization step) is disadvantageous since a loss of yield is to be expected due to the increased formation of oligomers. See discussion in the article by Mares and Sheehan in Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 1, 1978, 9-16
The principal object of this invention is to provide a process which can be effectively operated in process equipment of a smaller volume for the cyclization section (step b) as compared to processes of the current state of the art.
This object is achieved by employing the combination of conditions wherein the solvent used in step (a) is an aqueous medium, including water, and the xcex5-caprolactam yield obtained in step (a) is brought to at least 10%, calculated on the initial molar amount of the compound according to formula (1), and that the xcex5-caprolactam is separated from the aqueous mixture obtained from step (a) by extraction using an organic extraction agent, and with the aqueous mixture resulting from the extraction step, containing the primary amino compound, is then used as the feed into step (b).
The above results in the following process according to the invention preparing for xcex5-caprolactam, wherein, in a first step (a), a compound having the general formula:
O=CHxe2x80x94(CH2)4xe2x80x94C(O)xe2x80x94Rxe2x80x83xe2x80x83(1) 
wherein R is xe2x80x94OH, xe2x80x94NH2 or Oxe2x80x94Rxe2x80x2, and wherein Rxe2x80x2 is an organic group with 1 to 10 carbon atoms, and in an aqueous medium as solvent,
is contacted at an elevated pressure with ammonia and hydrogen in the presence of a hydrogenation catalyst to form a mixture of xcex5-caprolactam and primary amino compounds,
and wherein the yield to xcex5-caprolactam in step (a) is carried to a level of at least 10 molar%, calculated on the initial molar amount of said compound,
and extracting xcex5-caprolactam from said aqueous mixture obtained from step (a) with an organic extraction agent
to form an organic extractant solution of xcex5-caprolactam and a separate residual aqueous mixture,
followed by a separate second step (b) wherein said primary amino compounds in said residual aqueous mixture are further reacted to form xcex5-caprolactam.
By using an aqueous medium (including water) as the solvent in step (a) and by increasing the xcex5-caprolactam yield in step (a), xcex5-caprolactam can be advantageously separated from the reaction mixture prior to step (b) by extraction. Then, as a result of the separation of the xcex5-caprolactam prior to step (b), a smaller-sized volume of process equipment can be effectively used in step (b) while avoiding the drawbacks of the present state of the art.
A further advantage of the present invention is that a substantial part of the xcex5-caprolactam can be prepared at the relatively low temperature used in the first step (a). By contrast, in the process of U.S. Pat. No. 4,730,041, almost all of the xcex5-caprolactam is prepared in the second step (b) at relatively high temperatures, for example 300xc2x0 C. This is a significant temperature difference. By operating according to the present invention, the overall consumption of energy required to prepare one mol of xcex5-caprolactam of the process will be less than that of the present state of the art process.
Further, the fact that less xcex5-caprolactam is exposed to the higher temperature of the second step is also advantageous in that the level of impurities in the xcex5-caprolactam obtained is lowered. Moreover, at higher temperatures xcex5-caprolactam tends to react more readily to impurities than at lower temperature levels.
Another advantage of the present invention is that xcex5-caprolactam can be obtained in higher overall yields than was possible with the state of the art process as described in U.S. Pat. No. 4,730,041.
Examples of processes which yield more than 10% of xcex5-caprolactam in a process comparable to step (a) are generally not reported in the prior art, probably because at such higher yields xcex5-caprolactam oligomers can be formed. Oligomer formation is as a rule considered to be disadvantageous when the desired product is xcex5-caprolactam. However, we have now found that such oligomer formation in step (a) does not have to result in a reduction of the overall xcex5-caprolactam yield.
It has further been found that xcex5-caprolactam can be exclusively separated from an aqueous mixture containing 6-aminocaproic acid, 6-aminocaproamide and/or their respective oligomers. These primary amino compounds are the most important reaction products of step (a) and are the starting compounds for the further reaction to xcex5-caprolactam in step (b).
The extraction of xcex5-caprolactam from the aqueous mixture can be performed with any organic extraction solvent which is substantially immiscible with the aqueous mixture. By substantially immiscible is here meant that the mixture of organic extraction solvent and the aqueous mixture results in two segregated phases at the extraction temperature. Preferable the mutual solubility under the conditions of the extraction is not higher than 30 wt. % and more preferably less than 20 wt. %.
Examples of such solvents include ethers, for example methyl tert-butylether, aromatics, for example toluene, benzene and xylene and parafinic solvents, for example decaline. Preferably chlorinated hydrocarbons with 1 to 10 carbon atoms are used. Examples are dichloromethane, chloroform or 1,1,1-trichloroethane.
Examples of another class of extraction agents are phenol and alkyl phenols. A preferred class of alkyl phenols are those which have a boiling point higher than that of xcex5-caprolactam. Preferably, the alkyl phenol has a boiling point higher than the boiling point of xcex5-caprolactam, which is 270xc2x0 C. at 0.1 MPa. Alkyl phenols have high boiling points at atmospheric pressure. Therefore, in this context, the boiling points are advantageously compared at reduced pressures of, for example, 1.3 kPa (10 mmHg). Caprolactam has a boiling point of 140xc2x0 C. at 10 mmHg, while dodecyl phenol, for example, has a boiling point of 190xc2x0 C. at that pressure. By preference, the boiling point of the alkyl phenol is at least about 5xc2x0 C., and in particular, at least about 15xc2x0 C. above the boiling point for caprolactam at 1.3 kPa (10 mmHg). The upper limit for the boiling point of the alkyl phenol is about 400xc2x0 C. at 10 mmHg. Preferably, the alkyl phenol is chosen so as not to form an azeotropic mixture with xcex5-caprolactam.
The alkyl phenol is phenol substituted with one or more alkyl groups. The total number of carbon atoms of the alkyl group(s) is preferably between 6-25 and more preferably between 8-15. Examples of specific alkyl phenolic compounds include dodecyl phenol, octyl phenol, nonyl phenol, n-hexyl phenol, 2,4-diisobutyl phenol, 2-methyl-4,6-di-tert-butyl phenol, 3-ethyl-4,6-di-tert-butyl phenol, 2,4,6-tri-tert-butyl phenol, and mixtures of any thereof. U.S. Pat. No. 4,013,640 discloses additional alkyl phenols which may also be used, the complete disclosure thereof is hereby incorporated by reference. Other mixtures of alkyl phenols can also be used.
Most preferred extraction solvents are aliphatic or cycloaliphatic compounds having one or more hydroxyl groups. Such alcohol compounds have preferably 4-12 carbon atoms and more preferably 5-8 carbon atoms. Preferably one or two and more preferably only one hydroxyl group is present. Preferably hindered alcohols are used. A hindered alcohol is a compound in which the hydroxyl group is bonded to a xe2x80x94CR1R2R3 in which R1 and R2 are alkyl groups and R3 is an alkyl group or hydrogen. This is advantageous in a process in which the resulting aqueous phase are used as feed to prepare xcex5-caprolactam. Hindered alcohols are less susceptible to react to N-alkylation products of xcex5-caprolactam.
Examples of compounds having two hydroxyl groups are hexanediol, nonanediol, neopentylglycol, methyl-methylpropanediol, ethyl-methylpropanediol or butyl-methylpropanediol. Examples of compounds having one hydroxyl group are cyclohexanol, n-butanol, n-pentanol, 2-pentanol, n-hexanol, 4-methyl-2-pentanol, 2-ethyl-1-hexanol, 2-propyl-1-heptanol, n-octanol, iso-nonylalcohol, n-decylalcohol and mixtures of linear and branched C8-alcohols, mixtures of linear and branched C9-alcohols and mixtures of linear and branched C10-alcohols. Mixtures of the above mentioned alcohols can also be used. Preferred alcohols have a high affinity for xcex5-caprolactam, a lower boiling point than xcex5-caprolactam, a large density difference with water, commercially available, low mutual solubility with water and/or are biodegradable.
The extraction step is carried out at a temperature which is above the melting point of the organic extraction agent. The temperature of extraction can be generally between room temperature and about 200xc2x0 C.
The extraction step is carried out under a reduced pressure, but the actual pressure used is not critical. The pressure during the extraction step can be, for example, between about 0.1 MPa and about 2.0 MPa, and preferably, between about 0.1 MPa and about 0.5 MPa. The extraction step can be carried out in well known extraction apparatus, for example a counter current column or a series of mixer/settlers.
The extraction step yields an organic phase which, in general, may contain up to about 50 wt. % xcex5-caprolactam, along with between about 0 and about 15 wt. % water.
The starting compound according to formula (1) (the aldehyde compound) can itself be obtained by hydroformylation of the corresponding pentenoate ester, acid or amide as, for example, described for the ester in WO-A-9426688 and WO-A-9518089, and for the acid in, for example, WO-A-9518783, the disclosures of which are incorporated herein by reference. Preference is given to the 5-formylvalerate ester as the starting compound because this compound is at present more easily obtainable.
In formula (1), R is defined as one of the groupings xe2x80x94OH, xe2x80x94NH2 or xe2x80x94Oxe2x80x94Rxe2x80x2, and wherein Rxe2x80x2 is preferably an organic group with 1 to 20 carbon atoms. This organic group is an alkyl, cycloalkyl, aryl or aralkyl group. More preferably Rxe2x80x2 is an alkyl group. Examples of Rxe2x80x2 groups include methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, isobutyl, cyclohexyl, benzyl and phenyl. By preference Rxe2x80x2 is methyl or ethyl.
Step (a) can be performed by the generally known methods for reductive amination. The higher yield to xcex5-caprolactam in step (a) can be achieved by choosing conditions which are generally known to improve product yields. When starting from 5-formylvaleric acid the conditions as described in for example U.S. Pat. No. 4,730,040 can be applied and when starting from a 5-formylvalerate ester the conditions as described in U.S. Pat. No. 4,730,041 can be applied. Ammonia is preferably present at a molar excess with respect to the aldehyde compound. When 5-formylvalerate ester is the starting material, it is preferred to perform step (a) in the presence of an additional alcohol solvent and more preferably in the presence of the corresponding alcohol of the ester (Rxe2x80x2xe2x80x94OH). The presence of the alcohol improves the solubility of the 5-formylvalerate ester in the aqueous reaction mixture. The concentration of the alcohol is preferably between about 2 and 20 wt. % and more preferably between about 5 and 15 wt. %. The temperature in step (a) is preferably between about 50 and 150xc2x0 C. and the pressure is between about 0,5 and 20 MPa.
The hydrogenation catalyst comprises one or more of the metals choosen from the metals of groups 8-10 of the Periodic System of the Elements (New IUPAC notation; Handbook of Chemistry and Physics, 70th edition, CRC Press, 1989-1990) for example nickel, cobalt, ruthenium, platinum or palladium. Preference is given to Ru-, Ni- or Co-containing catalysts. In addition to Ru, Co and/or Ni the catalysts can also contain other metals for example Cu, Fe and/or Cr. The content of these additional metals may, for example, be up to 20 wt. %, based on the total metal content.
The catalytically active metals may optionally be employed on a carrier. Suitable carriers include for example aluminium oxide, silica, titanium oxide, zirconium oxide, magnesium oxide, carbon or graphite. Non-carried metals can also be used. An example of a non-carried metal is finely dispersed ruthenium. Preferred Ni- and Co-containing catalysts are Raney nickel and Raney cobalt optionally in combination with small amounts of another metal, for example Cu, Fe and/or Cr.
Most preferred are ruthenium-containing catalysts. High yields to xcex5-caprolactam in step (a) over a prolonged period of time are possible when using ruthenium-containing catalysts. Examples of possible ruthenium-containing catalysts are non-carried, or unsuported, metal catalysts, for example finely dispersed ruthenium, or ruthenium supported on a carrier, for example ruthenium on a carbon, alumina, graphite or TiO2 carrier.
Step (a) is desireably performed according to the preferred embodiment (described hereinbelow). It has also been found that higher overall yields to xcex5-caprolactam can then be achieved.
In its preferred embodiment, step (a) is performed in two separate substeps: substep (a1) and substep (a2). In substep (a1) the aldehyde compound corresponding to formula (1) is reacted with ammonia under non-hydrogenating conditions and in substep (a2) the reaction product obtained in substep (a1) is converted to xcex5-caprolactam and primary amino compounds under hydrogenating conditions in the presence of ammonia.
Substep (a1) is carried out under non-hydrogenating conditions. The term xe2x80x98non-hydrogenating conditionsxe2x80x99 means that the reaction conditions are such that, either no hydrogen is present or, if hydrogen is present, then the aldehyde compound according to formula (1) or a reaction product thereof is not, or is virtually not, reduced by the hydrogen. In general, non-hydrogenating conditions are realized by carrying out the first substep (a1) in the absence of a hydrogenation catalyst.
Variations in technique are possible. In one such embodiment of the present process, the hydrogen (which is needed in substep (a2)) can already be present in substep (a1). On the other hand if the hydrogenation catalyst is already introduced and present in this substep (a1), then non-hydrogenating conditions can nonetheless be achieved by avoiding the addition of hydrogen to the reaction mixture until after completing substep (a1). A third possible embodiment is that both hydrogen and the hydrogenation catalyst are absent from substep (a1).
The temperature in substep (a1) may be up to about 120xc2x0 C. and is preferably between about 0xc2x0 C. and 100xc2x0 C. More preferably the temperature is between about 20-100xc2x0 C. It has been found that the best results with regard to the overall yield obtained for primary amino compounds and xcex5-caprolactam are achieved when the conversion of the aldehyde compound in substep (a1) is more than 90%, preferably above about 99%. If the conversion is too low, this can result in an increase in the formation of, for example, the 6-hydroxycaproate ester (or acid or amide) and/or secondary amino compounds. Formation of these compounds will then negatively influence the overall process yield to xcex5-caprolactam.
As explained above, too short a contact or residence time in substep (a1) may result in undesirable by-product formation. The optimal residence or contact time at which the conversion of the aldehyde starting compound is virtually completed, will depend on the overall combination of reaction conditions, for example: temperature, concentration of reactants and method of mixing. Longer contact, or residence, times than are needed to achieve the above conversion are, of course, possible. The optimal residence time or contact time can be easily determined by the man skilled in the art.
Starting from the temperature and concentration range herein described, the residence or contact times, under normal mixing conditions, will generally preferably be more than about 5 seconds. Preferably, the residence or contact time will be less than about 2 minutes.
Substep (a1) is carried out in the presence of ammonia, and preferably a molar excess of ammonia is chosen such that the molar ratio ammonia:aldehyde compound is between 1:1 and 500:1 calculated on the starting amount of the aldehyde compound. Preferably this ratio is above about 5:1. If this ratio is too low the xcex5-caprolactam yield is negatively influenced. Preferably the molar ratio ammonia:aldehyde compound (aldehyde compound and its reaction products) in substep (a1) is between about 3:1 and 25:1 more preferably between about 5:1 and 15:1.
Water will be formed in substep (a1) as a reaction product of the reaction between the aldehyde compound and ammonia. Preferably substep (a1), and substep (a2) as well, are performed in the presence of at least 10 weight percent of water. The water content of the reaction mixture in substep (a1) is preferably between about 15-60 wt. % and more preferably between about 20-50 wt. %.
The concentration of the aldehyde compound, or more accurately, the concentration of the sum of aldehyde compound and its reaction products in step (a) or in substep (a1) is generally between about 1 and 50 wt. % and preferably between about 10 and 35 wt. %. High yields to xcex5-caprolactam can be advantageously achieved at these higher concentrations.
The pressure in substep (a1) is not critical. The pressure is generally equal or greater than the resulting equilibrium pressure of the liquid reaction mixture and the temperature employed.
Substep (a1) can be carried out in the presence of a catalyst, for example an acid ion exchanger or an acidic metal oxide catalyst, for example alumina or TiO2. Still, the conversion of the aldehyde starting compound in the first step also proceeds favorably in the absence of a catalyst. Because the overall yield to xcex5-caprolactam is not greatly influenced by the presence of a catalyst in the first step, such a catalyst is generally not used.
The process according to the invention can be performed batch wise or continuously. A large scale commercial process will preferably be performed continuously. For substep (a1), it is important that the reactants are sufficiently contacted at a certain temperature during a specified period of time optionally in the presence of a catalyst as described above. Any manner of contacting will usually suffice. For example a tube reactor with or without internal baffling or packing or a static mixer is a possible contacting unit for substep (a1). To control the temperature in substep (a1) it may be advantageous to use cooling devices, for example cooled walls or a cooling spiral placed in the contacting unit.
The above described ratios and concentrations and their preferred values for substep (a1) also apply for substep (a2) unless otherwise mentioned. Moreover the composition of the aqueous reaction mixture obtained in substep (a1) is by preference, directly and without substantial separation of any of the compounds of the mixture, used in substep (a2). This is advantageous because it results in a more simple process.
The reaction product obtained from substep (a1) is converted in substep (a2) to xcex5-caprolactam and the primary amino compounds under hydrogenating conditions in the presence of ammonia.
The primary amino compounds so obtained include 6-aminocaproamide, 6-aminocaproate ester and 6-aminocaproic acid. Oligomers which may be formed in step(a) can, in this invention, also be considered as primary amino compounds and as precursors to xcex5-caprolactam. The oligomers are for the greater part dimers of 6-aminocaproic acid or dimers of 6-aminocaproamide. Trimers and higher oligomers may also be formed.
When the aldehyde compound according to formula (1) is a 5-formylvalerate ester a mixture of xcex5-caprolactam, 6-aminocaproic acid, 6-aminocaproamide and none or a small amount of 6-aminocaproate ester and/or oligomers will be obtained in substep (a2). The hydrolysis of the ester-group mainly takes place in substep (a2). When the aldehyde compound is 5-formylvaleric acid a mixture of xcex5-caprolactam, 6-aminocaproic acid, possibly some 6-aminocaproamide and possibly some oligomers will be obtained in substep (a2).
By xe2x80x98hydrogenating conditionsxe2x80x99 it is understood in this invention that the reaction conditions are such that the intermediate reaction product(s) obtained in substep (a1) can be reduced by hydrogen. In general hydrogenation conditions are achieved when hydrogen and a hydrogenation catalyst are present. The hydrogenation catalyst has been described above.
The total pressure used in substep (a2) is generally between 0.5 and 20 MPa. The pressure is preferably between 0.5-10 MPa and more preferably between 1-5 MPa.
Substep (a2) is in general carried out at a temperature higher than about 40xc2x0 C. In general the temperature will be lower than about 200xc2x0 C. To achieve optimal overall yields to xcex5-caprolactam the temperature is more preferably between about 70 and 180xc2x0 C. Most preferably the temperature is above about 100xc2x0 C. because high yields to xcex5-caprolactam can be obtained.
The residence or contact time in substep (a2) should be sufficiently long so as to reduce virtually all the intermediate products formed in substep (a1) to the desired yield of xcex5-caprolactam and primary amino compounds. Operative residence or contact times are preferably between around about a half minute up to about a couple of hours. When the process is carried out batch-wise or in a continuously operated slurry reactor the contact or residence time respectively will generally be higher than the residence time when a continuously operated tube reactor is used.
Substep (a2) may be performed continuously in a fixed bed reactor in which the heterogeneous hydrogenation catalyst is present. An advantage of this reactor is that the reactants are easily separated from the hydrogenation catalyst. Another mode of operating substep (a2) is by use of one or more continuously operated contactors in series in which the hydrogenation catalyst is present as a well mixed slurry (slurry reactor). This manner of operation has the advantage that the heat of the reaction from substep (a2) can be easily controlled by, for example, a cooled feed or by way of internally placed cooling devices. Examples of specific and suitable slurry reactors are single or multiple-staged bubble columns or a gas lift-loop reactor or a continuously stirred tank reactor (CSTR). The slurry-hydrogenation catalyst can be separated from the reaction mixture after substep (a2) by, for example, using hydrocyclones and/or by filtration, for example by cake- or cross-flow filtration.
The catalyst concentration in substep (a2) may be choosen over a wide range. In a fixed bed reactor the amount of catalyst per volume will be high while in a slurry reactor this concentration will in general be lower. In a continuously-operated slurry reactor the weight fraction of catalyst (including the carrier) is typically between 0.1 and 30 weight % relative to the total content of the reactor. The weight fraction will for example depend on the use of a carrier and the kind of carrier.
The yield to xcex5-caprolactam in step(a) is in the process according to the invention above 10 wt. % and preferably higher than 20 wt. %.
The high yields to xcex5-caprolactam can be achieved by, for example, increasing the concentration of the reactants in substeps (a1) and (a2); increasing the residence time in step (a), or in substep (a2) when performing the two step reductive amination; increasing the temperature in substep (a1); and/or by using a ruthenium containing catalyst in step (a) (or substep (a2)).
Ammonia, hydrogen, the hydrogenation catalyst and the alcohol (if present) are preferably separated from the reaction mixture obtained in the reductive amination step (a) prior to the extraction according to the process of the invention. Hydrogen and part of the ammonia can be advantageously be separated by reducing the pressure and performing a gas/liquid separation. An example of such an operation is a flash operation performed at between ambient pressure and 0.5 MPa. The hydrogen and ammonia can advantageously be recycled to step (a).
In a subsequent step the alcohol (if present) can be separated. It has been found that it is advantageous to perform the cyclization step (b) in the pressence of less than 1 wt % and more preferably less than 0.1 wt % of alcohol. Thus when the resulting mixture from step (a) contains alcohol it is advantageous to separate this compound. It has been found that the presence of alcohol during the cyclization promotes the formation of the corresponding N-alkyl caprolactam, an undesired by-product. The presence of small quantities of these N-alkylated products, for example N-methyl xcex5-caprolactam, in final xcex5-caprolactam makes the xcex5-caprolactam less suitable for use as starting material for nylon-6 fibers. Because these N-alkylated products are difficult to separate from the final xcex5-caprolactam, it is favorable that they do not form or that their formation is minimized in the process according to the invention.
Separating the alcohol may be performed by any known method known to the man skilled in the art, for example by distillation or stripping, for example by steam stripping.
Step (b) may be performed in the gas phase as described in for example U.S. Pat. Nos. 4,599,199 or in 3,658,810 by contacting a (preferably concentrated) mixture as obtained in step (a) with overheated steam having a temperature between about 150-400xc2x0 C. at about atmospheric pressure. Such gas phase processes are advantageous because xcex5-caprolactam is obtained in a gaseous steam phase in which no oligomers are present. Separation of xcex5-caprolactam and oligomers can thus be avoided.
Preferably step (b) is performed in the liquid phase at super atmospheric pressures such as for example described in the aforementioned U.S. Pat. No. 4,730,040, WO-A-9600722 and in the above mentioned article of Mares and Sheehan. High yields of xcex5-caprolactam of high quality can be obtained with these liquid phase processes. More preferably step (b) is performed in the liquid phase as discussed below.
The concentration of ammonia in the liquid mixture employed in step (b) is preferably below about 5 wt. % and more preferably below about 3 wt. % and most preferably below about 1 wt. %. Higher concentrations of ammonia have a negative effect on the yield to xcex5-caprolactam per pass in a continuous process.
The concentration of xcex5-caprolactam and xcex5-caprolactam precursors in step (b) is preferably between 5 on 50 wt. % and more preferably between 10-35 wt. %.
The elevated temperature in step (b) is between about 200 and 350xc2x0 C. Preferably the temperature in step (b) is higher than 290xc2x0 C. because of higher yield per pass to xcex5-caprolactam.
The pressure in step (b) is preferably between 5.0 and 20 MPa. Normally this pressure will be greater than or equal to the resulting pressure of the liquid reaction mixture and the temperature employed.
Step (b) can be performed continuously in process equipment resulting in high and low rates of backmixing.
The xcex5-caprolactam can be separated from the reaction mixture obtained in step (b) by for example crystallization, extraction or by distillation. Preferably xcex5-caprolactam is separated by extraction in which the same extraction agents and conditions as described before can be applied. More preferably the effluent obtained in step (b) is subjected to the same extraction procedure as is used for the effluent of step (a) as described above. Prior to this extraction step it is prefered to separate part or all of the ammonia present in the aqueous mixture of step (b) in order to prevent a build up of ammonia in the process.
Extraction of xcex5-caprolactam from the effluent of step (b) is especially advantageous as compared to distillation separation when oligomers are also present in the aqueous mixture containing the xcex5-caprolactam. When using distillation more oligomers are usually formed and obtained in the residue of the distillation(s) in a high concentration. Because of these high oligomer concentrations and the solidification of the oligomers, fouling of the process equipment can occur, for example, pipes and other components. This disadvantage does not occur when extraction is used as the method for isolating xcex5-caprolactam.
Another advantage of the extraction procedure over distillation is that the amine compounds which can be present in the effluent of step (b) are not exposed to the high reboiler temperatures of the distillation. These high temperature conditions tend to induce formation of by-products and (more) oligomers. By using extraction as the method for isolating xcex5-caprolactam the exposure of the xcex5-caprolactam precursors to the high temperatures of the reboilers can be avoided or at least substantially reduced.
The xcex5-caprolactam may be purified by methods known for purifying xcex5-caprolactam obtained by Beckmann rearrangement. An example of a one method for purifying xcex5-caprolactam is described in U.S. Pat. No. 5,496,941.
A non restrictive example of an embodiment of the process starting from methyl 5-formylvalerate according to the invention is given in FIG. 1. The illustrated process is a schematic representation of the process equipment used in the below examples.
In FIG. 1 a mixture of methyl 5-formylvalerate/water/ammonia/methanol (1) is fed to the reductive amination reactor (A). Also fed to (A) is sufficient hydrogen (2). The effluent from the reductive amination reactor (A) is lead by line to vessel (B) wherein ammonia and methanol is separated by steam stripping. Part of the methanol is recovered via line (6) and the rest is recycled to the reductive amination stage (A) via line (5). The resulting reaction mixture is lead via line (7) to a counter current extraction column (C) and is then extracted with the extraction solvent through (8) to yield a extraction solvent stream (9) rich in xcex5-caprolactam and an aqueous mixture via line (11) rich in 6-aminocaproic acid, 6-aminocaproamide and oligomers. In vessel (E) water is first separated and removed from the mixture by distillation and recycled to the reductive amination stage via line (3). The resulting now concentrated aqueous mixture is fed via line (11xe2x80x2) to the cyclization reactor (F) resulting, in an effluent in line (12) rich in xcex5-caprolactam, but also containing some unconverted oligomers, plus 6-aminocaproic acid and 6-aminocaproamide. After separating ammonia via line (14), for instance, by steam stripping in vessel (G) the aqueous mixture (13) is recycled to the extraction column (C). Optionally the effluent of vessel (F) may be recycled to the steam stripper (B) via (12xe2x80x2). In this manner the feed to the extraction column (C) is concentrated and ammonia may be effectively separated by using less expensive and complicated process equipment. The xcex5-caprolactam extraction solvent mixture obtained in column (C) is supplied via line (9) to a separation unit (D) in which the organic solvent is separated from the xcex5-caprolactam by, for example, distillation, and xcex5-caprolactam is obtained through line (10). The extraction solvent itself, now poor in xcex5-caprolactam, is returned via line (8) to column (C). In the various recirculating streams, purges (not shown) will preferably be provided to overcome build up of contaminants and by-products.
The invention will now be elucidated with the following non-restricting examples.
The composition of the resulting mixtures of the experiments are sometimes expressed in mol percentages. The molar percentage of a component is represented by the molar fraction (xc3x97100%) of the molar amount of converted methyl 5-formylvalerate (M5FV) which contributes to that specific component. For example if the starting amount of M5FV is 100 mol and the resulting mixture contains 50 mol xcex5-caprolactam and 25 mol of dimers then the molar contribution to xcex5-caprolactam will be 50 mol % and the molar contribution to the dimers will be 50 mol % (totaling 100 mol %). When no oligomers such as dimers are present in the mixture the above molar percentages are the same as the molar yield as expressed below:             yield of component         ⁢    x    =                              mol          ⁢                      xe2x80x83                    ⁢          component          ⁢                      xe2x80x83                    ⁢          x          ⁢                      xe2x80x83                    ⁢          formed                ⁢                  xe2x80x83                            mol        ⁢                  xe2x80x83                ⁢        M5FV        ⁢                              xe2x80x83                    ⁢                      xe2x80x83                          ⁢        converted              *    100    ⁢    %  
Extraction Experiments